Electrostatic discharge is, for example, an electrical current impulse that, triggered by a great difference in potential, can flow through electrically conducting, but also through a normally electrically insulating material.
Integrated circuits (ICs) are often damaged by voltage peaks that overload active elements within the integrated circuit and can cause permanent damage. When, for example, a body having static electricity contacts metallic pins of an IC, it will discharge a high voltage through the metallic pins of the IC, that may result in its internal circuit being damaged. Electrostatic discharge can also cause an electrical system to lose its effectiveness.
An ESD protective circuit can prevent damaging effects of electrostatic discharge. At the same time, an ESD protective circuit must be able to withstand the energy of the electrostatic discharge in order not to be damaged itself. Furthermore, an ESD protective circuit should only intervene if there is electrostatic discharge.
In normal operation of the integrated circuit, i.e. when there is no electrostatic discharge at the input/output contact of the IC, it is advantageous for reasons of current consumption of the integrated circuit when no or only a low current flows through the input/output contacts of the IC. Low currents through the input/output contacts will only be demanded if the circuit to be protected is energized, i.e., in operation. Signals from sensors that are either supplied from a different supply voltage than, for example, evaluating ICs to be protected, or signals in which overvoltages or undervoltages at the input/output contacts of the evaluating IC may arise due to shifts of reference potentials on long lines, may allow the input voltages of the IC to exceed an upper supply voltage (VDD) or fall below a lower supply voltage (VSS). Thus, an ESD protective circuit should be dimensioned such that it comprises only low input currents at input voltages to be tolerated slightly over the upper supply voltage (VDD) or slightly below the lower supply voltage (VSS).